flash: fix stm32 failed probe using incorrect flash size

flash: fix stm32 failed probe using incorrect flash size

This fixes an issue if the device is manually probed after the initial probe
fails due to being unable to read flash size register. In this situation the
driver assumes the user has overridden the flash size when infact this may
not be the case.

It also seems on the older stm32f1 devices the flash register is not readable
when locked, this does not seem to apply to the newer parts - f0, f3, f4.

Change-Id: I125f872fcb2d962ca6705f97b62d957e2b31303b
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1187
Tested-by: jenkins
Reviewed-by: Johan Almquist <johan.almquist@assaabloy.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>